Powder-metallurgical composition having good soft magnetic properties

ABSTRACT

The invention relates to an iron-based powder composition which, in addition to a substantially non-alloyed Fe-powder, comprises Sn and P, optionally lubricant and at most 1.0% by weight of impurities. In the composition, Sn and P are present as an SnP-alloy in powder form, or else Sn is present in the form of a metallic powder and P is present in the form of a ferrophosphorous powder, the Sn-content, based on the total iron-based powder composition, being at least 4.5% by weight, and the individual particles, which contain Sn and P, being present as particles substantially separate from the particles in the non-alloyed Fe-powder. Finally, Sn and P may also be present as an SnP-alloy in powder form, and Sn may also be present as a metallic powder. This composition may optionally also contain P as a ferrophosphorous powder.

The present invention relates to an iron-based powder compositioncontaining Sn and P for manufacturing components with stringent demandsin respect of soft magnetic properties and low eddy current losses.

One of the major advantages gained from powder-metallurgical manufactureof components as compared with conventional techniques is that itpermits manufacturing components in long series with high dimensionalaccuracy. In such manufacture, an iron base powder is mixed e.g. withadditions of pulverulent alloying substances and a lubricant. Thealloying substances are added to give the finished component the desiredproperties, whilst the lubricant is added primarily to reduce the toolwear when compacting the powder mixture. The compacting of the powdermixture into the desired shape is followed by sintering.

Powder-metallurgical manufacture of components for soft magneticpurposes is today performed primarily by compacting and high-temperaturesintering, meaning temperatures above 1150° C. High-temperaturesintering is relied on above all since it is known that the softmagnetic properties are improved when the sintering temperature israised. It is above all the particle growth, but also such factors as amore homogeneous distribution of alloying substances and higher densitythat entail enhanced soft magnetic properties in these materials ascompared with materials sintered at lower temperatures.

The major iron-based tonnage for soft magnetic purposes is manufacturedwith the addition of Si, both to enhance the soft magnetic propertiesand to increase the resistivity so as to reduce the eddy current lossesin AC applications. Powder-metallurgical manufacture of Si-alloyedmaterials necessitates high-temperature sintering, since otherwise Siwould oxidise and not be dissolved into the iron. High-temperaturesintering however results in substantial shrinkage during sintering,which gives rise to difficulties in maintaining the dimensional accuracyon the components.

Components for soft magnetic purposes can also be manufactured in powdermetallurgy by adding P to iron-based materials. The addition of Penhances the soft magnetic properties as compared with pure Fe and alsoimproves the resistivity to some extent, that is, reduces the eddycurrent losses in AC applications. Moreover, the process technique issimple in that the components can be sintered in a belt furnace wherethe temperature is maximised to about 1150° C. P-alloyed materials, onthe other hand, have considerably lower resistivity than today'sSi-alloyed materials, both after sintering in a belt furnace and aftersintering at a high temperature (t>1150° C.).

The object of the present invention therefore is to provide aniron-based powder composition which after compacting and sinteringexhibits

improved soft magnetic properties as compared with currently knowniron-based powder-metallurgical materials;

high resistivity resulting in low eddy current losses.

Moreover, this powder composition should after compacting and sinteringexhibit

properties similar to those achieved with high-temperature sintering ofcurrently known iron-based powder-metallurgical materials when sinteringis performed in a belt furnace, i.e. at a maximum temperature of about1150° C.;

small dimensional change.

According to the invention, the desired properties can be obtained bymeans of an iron-based powder composition which, in addition to asubstantially non-alloyed Fe-powder, comprises Sn and P, optionallylubricant and at most 1.0% by weight of impurities, wherein

a) Sn and P are present as an SnP-alloy in powder form, or wherein

b) Sn is present in the form of a metallic powder and P is present inthe form of a ferrophosphorous powder, Fe₃ P, the Sn-content, based onthe total iron-based powder composition, being at least 4.5% by weightand the individual particles, which contain Sn and P, being present asparticles substantially separate from the particles in the non-alloyedFe-powder, or wherein

c) Sn and P are present as an SnP-alloy in powder form, and Sn isadditionally present as a metallic powder, and wherein, optionally, P isalso present as a ferrophosphorous powder Fe₃ P.

In powder compositions according to Alternatives a) and c) above, theSn-content may suitably range between 1.0 and 15.0% by weight and theP-content between 0.2 and 1.5% by weight. Preferably, the Sn-contentranges between 2.0 and 12.0% by weight and the P-content between 0.3 and1.2% by weight based on the total weight of the composition. The contentof impurities preferably is at most 0.5%.

In powder compositions according to Alternative b) above, the Sn-contentmay suitably range between 4.5 and 15% by weight, preferably between 5and 8% by weight, based on the total weight of the iron-based powdercomposition.

To obtain the required Sn- and P-contents in the powder composition, anaddition is made, e.g. of Sn and P as a powder of an SnP-alloycontaining Sn and P in such proportions that the desired alloyingcontents are obtained in the sintered component.

Preferably, the particle size distribution is such that the main portionof the particles of the SnP-alloy have a size below 150 μm. Also when Snis added as a metal powder, the particle size distribution suitably issuch that the main portion of the particles have a size below 150 μm,while P is added as ferrophosphorous powder having a P-content of 12-17%by weight and such a particle size distribution that the main portion ofthe particles have a size below 20 μm. Further, the required Sn- andP-contents can be adjusted in the powder composition by adding anSnP-alloying powder with the indicated particle size and also Sn and/orP. In this case too, a powder of metallic Sn, an SnP-alloy andferrophosphorus having the indicated particle sizes are also added.

It is previously known, for instance from JP 48-102008, that Sn may beincluded in compacted and sintered iron-based powder materials. Thisknown powder material may optionally also contain P which, however, thenis not in the form of Fe₃ P.

EP 151,185 A1 describes the addition of Sn as an oxide powder which,after compacting and sintering, yields a material that is stated to bean improvement over previously known materials. According to this patentspecification, there is also obtained a certain further improvement ofthe properties of this material when phosphorus in the form of Fe₃ P isadded. However, according to this publication an addition of Fe₃ P,together with a pure powder of metallic Sn, does not provide an overallimprovement of the soft magnetic properties and the resistivity incompacted and sintered iron-based powder materials as compared with thecase where Fe₃ P is not added. The resistivity is certainly improved,but at the same time the permeability is reduced. These results do notagree with those obtained with the present invention when a powder ofmetallic Sn and ferrophosphorus are added to a substantially non-alloyedFe-powder, the Sn-content in the present compositions being suitablyabove 4.5% based on the weight of the total iron-based powdercomposition. It has further been surprisingly found in conjunction withthe present invention that when Sn and P are added as an SnP-alloy inpowder form to iron-based powder compositions, there is obtained aftercompacting and sintering not only an essential improvement of the softmagnetic properties and the resistivity as compared with an addition ofa pure Sn-powder, but it is also possible to achieve clearly improvedmechanical properties, such as tensile strength. It is therefore notnecessary to add Sn in the form of a chemical compound of the typedisclosed in EP 151,185 A1 in order, optionally together with P, toachieve improved properties in the compacted and sintered component.Moreover, the invention according to EP 151,185 A1 involves acomplicated process technique as compared with the options according tothe present invention, since the material must undergo an additionalannealing process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a, 1b, and 1c show the relationship between phosphorous contentand permeability, coercive force, and resistivity, respectively, in oneexample of the invention.

FIGS. 2a, 2b, and 2c show the relationship between tin content andpermeability, coercive force, and resistivity, respectively, in anotherexample of the invention.

FIGS. 3a, 3b, and 3c show the relationship between tin content andpermeability, coercive force, and resistivity, respectively, in anotherexample of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be described in more detail hereinafter in someExamples.

EXAMPLE 1

Five iron-based powder compositions (A, B, C, D, E) were manufactured byadding five different SnP-alloying powders with varying Sn/P-ratios, toan iron powder with a low content of impurities.

The reference materials employed were two known iron-basedpowder-metallurgical materials commonly used in soft magneticapplications, viz. Fe-3% by weight Si and Fe-0.45% by weight P as wellas an Fe-5% by weight Sn-material. The nominal chemical compositionappears from Table 1 below.

                  TABLE 1                                                         ______________________________________                                        Nominal chemical composition of the materials tested.                                 Chemical composition (%)                                              Material  Sn      P          Si    Fe                                         ______________________________________                                        A         5.0     0.45       --    Balance                                    B         5.0     0.60       --    Balance                                    C         5.0     0.80       --    Balance                                    D         5.0     1.20       --    Balance                                    E         5.0     1.60       --    Balance                                    Ref. 1    --      --         3.0   Balance                                    Ref. 2    --      0.45       --    Balance                                    Ref. 3    5.0     --         --    Balance                                    ______________________________________                                    

These powders were admixed with 0.6% Kenolube as lubricant, and aftermixing test pieces were compacted at 600 MPa. Sintering was performed at1250° C. for 30 min in a reducing atmosphere (hydrogen gas). Thereference materials were sintered for 60 min.

After sintering, the properties permeability, coercive force andresistivity were measured, as illustrated in FIGS. 1a, 1b and 1c. Asappears from these Figures, there is achieved within the content range0.2-1.5% by weight P, which is the selected content range for P in thepresent invention, an improved combination of the propertiespermeability, coercive force and resistivity than what is previouslyknown. The upper limit for P, which is 1.5% by weight, is explained byreduced permeability and lower coercive force at higher P-contents ascompared with the known reference materials. The advantage of highresistivity then does no longer make up for the poorer soft magneticproperties (lower permeability, higher coercive force). The lower limitfor P, which is 0.2% by weight P, is explained by a reduction ofpermeability, coercive force and resistivity, such that a combination ofthese properties cannot be considered superior to the known techniquewhen the P-content is below 0.2% by weight. In the preferred contentrange, i.e. 0.3-1.2% by weight P, the permeability is higher and thecoercive force is lower in the inventive material as compared with thereference materials Fe-3% Si, Fe-0.45% P and Fe-5% Sn. The resistivityis similar for the inventive material as for Fe-3% Si, while Fe-0.45% Pand Fe-5% Sn have lower resistivity. In the preferred content range forP, i.e. 0.3-1.2% by weight P, there is shown an improved combination ofthe properties permeability, coercive force and resistivity achievablewith the inventive material as compared with the known technique.

EXAMPLE 2

Five iron-based powder compositions (F, G, H, I, J) were prepared byadding five different SnP-alloying powders with varying Sn/P-ratios, toan iron powder with a low content of impurities. The same referencematerials as in Example 1 were used. The nominal chemical compositionappears from Table 2 below.

                  TABLE 2                                                         ______________________________________                                        Nominal chemical composition of the materials tested.                                 Chemical composition (%)                                              Material  Sn      P          Si    Fe                                         ______________________________________                                        F         2.0     0.45       --    Balance                                    G         5.0     0.45       --    Balance                                    H         8.0     0.45       --    Balance                                    I         10.0    0.45       --    Balance                                    J         15.0    0.45       --    Balance                                    Ref. 1    --      --         3.0   Balance                                    Ref. 2    --      0.45       --    Balance                                    Ref. 3    5.0     --         --    Balance                                    ______________________________________                                    

These powders were admixed with 0.6% Kenolube as lubricant, and aftermixing test pieces were compacted at 600 MPa. Sintering was performed at1250° C. for 30 min in a reducing atmosphere (hydrogen gas). Thereference materials were sintered for 60 min.

After sintering, permeability, coercive force and resistivity weremeasured in a similar way as in Example 1. As appears from FIGS. 2a, 2band 2c, there is achieved within the content range 1.0-15.0% by weightSn, which is the selected content range for Sn in the present invention,an improved combination of the properties permeability, coercive forceand resistivity than is previously known. The upper limit for Sn, whichis 15.0% by weight, is explained by the permeability showing a steeplydeclining trend, and the advantage of a very high resistivity thencannot make up for the drastically reduced permeability at higherSn-contents. The lower limit for Sn, which is 1.0% by weight, isexplained by too low a resistivity at lower Sn-contents which no longermakes up for the positive contribution in permeability and coerciveforce achievable even by small amounts of Sn. In the preferred contentrange, i.e. 2.0-12.0% by weight Sn, the permeability is higher and thecoercive force is lower than for all three reference materials. Theresistivity is similar for the inventive material and Fe-3% Si and Fe-5%Sn, while it is lower for Fe-0.45% P.

Within the preferred content range for Sn, i.e. 2.0-12.0% by weight Sn,there is shown a considerably improved combination of the propertiespermeability, coercive force and resistivity achievable with theinventive material as compared with the known technique.

EXAMPLE 3

Five iron-based powder compositions (K, L, M, N, O) were prepared byadding 0.45% by weight P in the form of a ferrophosphorous powder, Fe₃P, and different contents of Sn in the form of a metal powder, to aniron powder with a low content of impurities. The reference materialsused were the same as in Example 1. The nominal chemical compositionappears from Table 3 below.

                  TABLE 3                                                         ______________________________________                                        Nominal chemical composition of the materials tested.                                 Chemical composition (%)                                              Material  Sn      P          Si    Fe                                         ______________________________________                                        K         2.0     0.45       --    Balance                                    L         5.0     0.45       --    Balance                                    M         8.0     0.45       --    Balance                                    N         10.0    0.45       --    Balance                                    O         15.0    0.45       --    Balance                                    Ref. 1    --      --         3.0   Balance                                    Ref. 2    --      0.45       --    Balance                                    Ref. 3    5.0     --         --    Balance                                    ______________________________________                                    

These powders were admixed with 0.6% Kenolube as lubricant, and aftermixing test pieces were compacted at 600 MPa. Sintering was performed at1250° C. for 30 min in a reducing atmosphere (hydrogen gas). Thereference materials were sintered for 60 min.

After sintering, permeability, coercive force and resistivity weremeasured, as illustrated in FIGS. 3a, 3b and 3c. As appears from theseFigures, the results obtained are similar to those obtained when Sn andP are added as an SnP-alloying powder.

It is evident to those skilled in the art that similar results can beachieved if the substantially non-alloyed iron powder is admixed with apowder consisting of a combination of metallic Sn and SnP, andoptionally P in the form of Fe₃ P.

It has also been found that when compositions according to the inventionare subjected to sintering in a belt furnace (at a temperature <1150°C.), similar soft magnetic properties are achieved in the sinteredproduct as are obtained from high-temperature sintering of currentlyknown materials. Furthermore, the sintered products prepared from apowder according to the invention exhibit a considerably smallerdimensional change than these known materials.

The following Example gives a comparison between known compositions andcompositions according to the invention.

EXAMPLE 4

A iron-based powder material was prepared with the nominal chemicalcomposition 5% Sn and 0.45% P, where Sn and P were added as anSnP-alloying powder, the remainder being Fe. The references used wereFe-3% Si and Fe-0.45% P. In all three powders, 0.6% Kenolube was admixedas lubricant, and after mixing test pieces were compacted at 600 MPa.Sintering was performed at 1120° C. for 30 min in reducing atmosphere(hydrogen gas) for the inventive powder, while the reference materialswere sintered at 1250° C. for 60 min in the same type of atmosphere.Moreover, Fe-0.45% P was also sintered at 1120° C. under otherwise thesame conditions as at the higher temperature. In Table 4 below, theresults after sintering are compared.

                                      TABLE 4                                     __________________________________________________________________________    Sintering conditions and properties of the tested materials after             sintering.                                                                              Sintering                                                                            Dimensional                                                            temperature                                                                          change Density                                                                            B-max                                                                             Hc  μ-                                                                            Resistivity                           Material  time, atm.                                                                           %      g/cm.sup.3                                                                         T   A/CM                                                                              max                                                                              μ ohm cm                           __________________________________________________________________________    Fe-5% Sn-0.45% P                                                                        1120° C.                                                                       0.21  7.20 1.30                                                                              0.83                                                                              4800                                                                             43                                              30', H.sub.2                                                        Fe-3% Si (ref.)                                                                         1250° C.                                                                      -1.25  7.21 1.34                                                                              0.79                                                                              4300                                                                             47                                              60', H.sub.2                                                        Fe-0.45% P (ref.)                                                                       1250°                                                                         -0.60  7.40 1.40                                                                              0.75                                                                              5600                                                                             22                                              60', H.sub.2                                                        Fe-0.45% P (ref.)                                                                       1120° C.                                                                      -0.30  7.25 1.35                                                                              0.98                                                                              4900                                                                             23                                              60', H.sub.2                                                        __________________________________________________________________________

As appears from the Table, the properties of the inventive material areequivalent to those of the best reference material although sinteringwas performed at a higher temperature for two of the reference materialsand, moreover, for a longer time for all three reference materials.Furthermore, the powder material according to the invention exhibits aconsiderably smaller dimensional change than do the references sinteredat 1250° C. To sum up, it can be stated that the invention complies withthe objective set, and in practice is most useful, since belt-furnacesintering can be used for many soft magnetic applications which normallyrequire high-temperature sintering with consequent difficulties, e.g. inrespect of dimensional accuracy. Still higher demands on soft magneticproperties are met by high-temperature sintering of a powder compositionaccording to the present invention, as described in Examples 1, 2 and 3above.

I claim:
 1. An iron-based powder composition which, in addition to asubstantially non-alloyed Fe-powder, comprises Sn and P, optionallylubricant and at most 1.0% by weight of impurities, the compositionbeing selected from first and second powder compositions wherein:a) inthe first powder, Sn and P are present as an SnP-alloy in powder form,the composition including 1.0-15.0% by weight Sn and 0.2-1% by weight P;or b) in the second powder, Sn and P are present as an SnP-alloy inpowder form, and, in addition, Sn is present as a metallic powder, andoptionally, P is also present as ferrophosphorous powder, thecomposition including 1.0-15% by weight Sn and 0.2-1.5% by weight P. 2.The powder composition as claimed in claim 1, wherein said powdercomposition is composition (a) or (b) and said powder includes 2.0-12.0%by weight Sn and 0.3-1.2% by weight P.
 3. The powder composition asclaimed in claim 1, wherein said powder composition is composition (a)or (b) and most of the SnP-alloy powder has a particle size below 150μm.
 4. The powder composition as claimed in claim 1, wherein said powdercomposition is composition (b) and the ferrophosphorus powder has a Pcontent of 12-17% by weight and most of the ferrophosphorus powder has aparticle size below 20 μm.
 5. The powder composition as claimed in claim1, wherein the composition has a magnetic permeability of at least 4000μ, a coercive force Hc of at least 0.4 A/cm and a resistivity of atleast 40 μ-ohm.cm.
 6. The powder composition as claimed in claim 1,wherein the composition is Si-free.
 7. The powder composition as claimedin claim 1, wherein the composition has a magnetic permeability of atleast 6000 μ, a coercive force Hc of at least 0.4 A/cm and a resistivityof at least 40 μ-ohm.cm.